If the clay is at all sensitive, small movements can rapidly turn into big ones. Given all the little details that govern undrained strength*, you probably need a higher FS to be comfortable than if you were dealing with dry material or rolled fill. (In good material, FS=1.1 can mean very low probability of failure. In others, FS=1.5 shouldn't make you feel warm and fuzzy.)
I'd sure hate to rely on piezos to be the main trip wire, especially with their slow response in clay, with the slightest bit of air in the porous stone the space between stone and diaphragm, even with VW piezos. At minimum, I would probably ask for stakes at the toe of the slope, to be shot periodically (both vertical and horizontal). Any significant heave or horiz displacement starts a rest period (plus frequent visual monitoring). Inclinometers would be good also, although a state DOT may balk.
You've seen Chuck Ladd's Terzaghi Lecture, "Stability Evaluation During Staged Construction"? If not, see ASCE JGE April 1991. It is right up your alley. It is the best single reference I know for undrained strength of clays.
I would also be inclined to put in disclaimers writ large, emphasizing the uncertain nature of subsurface conditions, particularly undrained shear strength, and the importance of monitoring to make certain that designs assumptions are met...Unfortunately, those things are usually managed by bean counters, not by people that understand there is a difference between geotechnical engineering and doing QC on toner cartridges.
The crack and slump in the photo appeared one morning. All was fine the night before.
*OCR
Rate effects
Mineralogy
Small changes in void ratio
Degree of saturation
Rotation of principal stresses
Assumed Nkt for CPT
Adjustment factor for VST